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-# Database helpers
-
-There are a number of useful helper modules defined in `/lib/gitlab/database/`.
-
-## Subquery
-
-In some cases it is not possible to perform an operation on a query.
-For example:
-
-```ruby
-Geo::EventLog.where('id < 100').limit(10).delete_all
-```
-
-Will give this error:
-
-> ActiveRecord::ActiveRecordError: delete_all doesn't support limit
-
-One solution would be to wrap it in another `where`:
-
-```ruby
-Geo::EventLog.where(id: Geo::EventLog.where('id < 100').limit(10)).delete_all
-```
-
-This works with PostgreSQL, but with MySQL it gives this error:
-
-> ActiveRecord::StatementInvalid: Mysql2::Error: This version of MySQL
-> doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery'
-
-Also, that query doesn't have very good performance. Using a
-`INNER JOIN` with itself is better.
-
-So instead of this query:
-
-```sql
-SELECT geo_event_log.*
-FROM geo_event_log
-WHERE geo_event_log.id IN
-    (SELECT geo_event_log.id
-     FROM geo_event_log
-     WHERE (id < 100)
-     LIMIT 10)
-```
-
-It's better to write:
-
-```sql
-SELECT geo_event_log.*
-FROM geo_event_log
-INNER JOIN
-  (SELECT geo_event_log.*
-   FROM geo_event_log
-   WHERE (id < 100)
-   LIMIT 10) t2 ON geo_event_log.id = t2.id
-```
-
-And this is where `Gitlab::Database::Subquery.self_join` can help
-you. So you can rewrite the above statement as:
-
-```ruby
-Gitlab::Database::Subquery.self_join(Geo::EventLog.where('id < 100').limit(10)).delete_all
-```
-
-And this also works with MySQL, so you don't need to worry about that.